JP2647366B2 - Image processing device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image processing apparatus, and more particularly, to an image processing apparatus that modulates a recording beam such as a laser beam according to a digital image signal.

[Prior Art] Today, laser beam printers are widely used due to their high image quality and high speed. FIG. 3 is a diagram showing the basic configuration of a conventional laser beam printer. In the figure, bit ON
The image input signal represented by the / OFF level is amplified by the laser driver circuit and turned on / off for laser unit lighting control.
It is converted to an FF level signal LDR. In this example, the laser unit is a semiconductor laser. The light emitted from the semiconductor laser is a collimator lens, a cylindrical lens,
An image is formed through an optical system including an imaging lens and a mirror so that the spot diameter is constant in any axial direction on the drum surface. On the other hand, the scanner motor driven by the scanner driver circuit rotates the decahedral mirror to scan the laser beam in the drum axis direction. Thus, an electrostatic latent image is formed on the surface of the photosensitive drum according to the image input signal. Then, the toner (black) is deposited on this portion by the subsequent development process, and thus the electrostatic latent image is visualized. In this case, the change amount of the drum surface potential is determined by the irradiation time of the laser beam and the beam intensity (power).
In the above-described laser beam printer, since the irradiation width per dot is constant and the beam intensity is constant, only a binary black-and-white image can be reproduced. Therefore, when reproducing a continuous density image such as a photograph, halftone expression is performed by the so-called dither method, but in any case, since it is a set of black and white dots, it is sufficient as a continuous density expression method. I can not say that.

As a method of improving the above-mentioned drawbacks of the halftone expression,
For example, it is conceivable to use a pulse width modulator to express the laser light as a continuous time difference of the pulse width according to the density. FIG. 4 (a) shows a block diagram of a possible pulse width modulator. Here are represented by the density data VD ₀ to VD ₅ 6 bit parallel image, as shown in FIG. (B), the image density by bit combination of a total of 64 combinations consisting of 0-63, for example, "0" = White density, expressed as "63" = black density. This image data is converted to a corresponding DC level by the D / A converter 1, and the conversion level is white = 0V and black = 5V. The converted DC level signal DA is input to one input terminal of a comparator (CMP) 3, and the other input terminal of the comparator 3 receives a triangular wave signal SAW generated by a signal generator (SG) 2. . In this way, the level of the triangular wave signal SAW and the level of the image signal DA are compared by the comparator 3 and converted into a pulse width modulation signal Pw as shown in FIGS. 4 (c) and 4 (d). That is, when the image signal is close to the white level, the pulse width of the signal Pw increases, and the laser diode LD is turned on for a relatively long time. When the level is close to the black level, the pulse width becomes narrow, and the laser diode LD is turned on for a relatively short time. By doing so, the bit parallel data is converted into an image signal Pw whose pulse width changes continuously, so that the gradation expression becomes smooth and the output characteristics of a photograph and the like are improved.

However, the image forming apparatus described above cannot easily solve various actual effects on the gradation of a reproduced image. For example, if the gradation density characteristics of the input image data are not linear in nature, they must be corrected in a complicated manner on the pulse width modulation system, which complicates the device structure and increases the cost. There was a disadvantage. Even if the gradation density characteristics of the input image data are linear, the same applies if there are variations in the sensitivity characteristics of the photosensitive drum, variations in the developer density and conditions during development, and variations in the conditions during transfer to paper. However, it is extremely difficult to correct such an image on a pulse width modulation system, and it is rather difficult to make the drum sensitivity characteristics and development characteristics uniform. At present, we are trying to solve this problem.

[Problems to be Solved by the Invention] The present invention has been made in view of the conventional example described above, and can faithfully reproduce the density of an original image and perform density correction caused by differences between models and apparatuses. It is an object of the present invention to provide an image processing apparatus capable of obtaining a high-gradation, high-resolution image.

[Means for Solving the Problems] In order to achieve the above object, an image processing apparatus of the present invention has the following configuration. Input means for inputting a digital image signal whose image density is represented by a plurality of bits; converting means for converting the digital image signal into an analog image signal; and analog of a predetermined cycle having an extreme value at substantially the center of one cycle. Pattern signal generation means for generating a pattern signal; modulation signal generation means for comparing the analog image signal and the analog pattern signal to generate a pulse width modulation signal for pulse width modulation of a recording beam; Control means for controlling the intensity of the recording beam in accordance with a value obtained by correcting the density of the signal.

[Operation] In the above configuration, a digital image signal whose image density is represented by a plurality of bits is input, and the input digital image signal is converted into an analog image signal. This analog image signal is compared with an analog pattern signal of a predetermined cycle having an extreme value at substantially the center of one cycle to generate and record a pulse width modulation signal for pulse width modulation of the recording beam, and input the pulse. The intensity of the recording beam is controlled according to the value obtained by correcting the density of the digital image signal.

[Embodiment] As one means for solving this problem, for example, the image forming apparatus of the embodiment shown in FIG.
VD ₀ to VD beam time width determining unit 1, 2, 3 to determine the lighting time width of the sub connexion light beam _5, the beam intensity determination means 4 for determining the intensity of the sub connexion light beam on the same density data, Five,
And light beam generating means (transistor Q and laser diode LD) for generating a light beam having the intensity Il of the output of the beam intensity determining means by the lighting time width Pwl of the output of the beam time width determining means.

In configuration of the Figure 1, 6-bit image data VD ₀ to VD ₅ image is converted into an analog image signal DA in proportion to the concentration of D / A converter 1, the image signal DA is generated from the signal generator 2 The signal is input to the comparator 3 together with the triangular wave signal and is compared. And the comparison output of the comparator 3 is HI
During the level, the transistor Q is turned on, during which time the laser diode LD is turned on. Meanwhile, the same density data VD ₀ to VD ₅ is, for example, inputted as the concentration data VD ₀ to VD ₅ of address data ROM5 storing the specified concentrations correction table to correct the non-linear component, whose output the terminal correction data R ₀ to R ₅ of the 6 bits are read. The correction data R _{0 to} R ₅ are converted by a D / A converter 6 into a current control signal Vc proportional to the correction data,
Determine the constant current value when driving the LD. In this way, the two major factors (beam lighting width and beam power) that determine the density of the output pixel are handled independently of each other, and the object of the present invention is achieved.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is a block diagram showing a density converter of the image forming apparatus according to the embodiment. Note that the same components as those in FIG. 4 (a) are denoted by the same reference numerals and description thereof is omitted. In the figure, LD is a laser diode, which is driven ON / OFF by a switching transistor Q. The peak current value Il flowing through the laser diode LD can be determined by the constant current source circuit 4. The constant current value Il can be freely set by setting the voltage level signal Vc at the control terminal of the constant current source circuit 4. Can be controlled. FIG. 2 (a) shows the input / output Vc.
Here is an example of the -Il characteristic. Here, when the level signal Vc is high, a relatively large current flows to the laser diode LD to increase the beam power, and when the level signal Vc is low, a relatively small current flows to the laser diode LD to reduce the beam power. It is in. Thus, if the HI level of the pulse width at the base of the switching transistor Q is applied,
The peak current value Il and the pulse width Pw as shown in FIG.
A drive current waveform of l is obtained. Reference numeral 6 denotes a D / A converter, which outputs an analog level signal VC proportional to the read signals _{R0 to} R5 of the ROM ₅ . 5 ROM der connexion, predetermined conversion output R ₀ to R ₅ is read to the input bit parallel image signals VD ₀ to VD _5. Moreover, this conversion can take various forms depending on the purpose. FIG. 2 (b) and (c)
Shows an example of the conversion table. Table A is a case of outputting directly the input data VD ₀ to VD _5,
Since a strong peak current Il is applied to image data having a black density, white on a black background is well represented. Table A 'is the opposite. Table B is an example in which the linearity of the conversion is lost, and the white output is enhanced for the intermediate density data. Table C is the opposite. Of course, the above relationship differs depending on whether development is performed with black toner (+) or white toner (-).
It can be freely determined depending on the relationship with various configurations of the image forming apparatus. Laser power Il is constant with respect to the image data VD ₀ to VD ₅ In further table D. This corresponds to a case where no density correction is performed in the apparatus of this embodiment. Similarly, in the case of the table E, the laser power Il is increased at the intermediate density data, and in the case of the table F, the reverse is true.

FIG. 2D shows the relationship between the beam lighting pulse width Pwl and the surface potential Vd on the photosensitive drum. The potential of the portion of the photosensitive drum irradiated with the laser beam for a certain area becomes relatively low, and is substantially inversely proportional to the irradiation pulse width if the beam power is constant. However, since this surface potential Vd is a function of the beam power along with the beam lighting time width, one can easily correct the nonlinearity of the other, or force the nonlinearity. become.

In the above-described embodiment, the case where the reproducibility of the image density is improved has been described. On the contrary, it is also possible to use the positive density control for emphasizing or weakening a certain density. Of course, to achieve such multiple purposes, ROM5
, A plurality of conversion tables may be prepared and switched by the selection means.

In the present embodiment, a triangular wave signal is used for pulse width modulation, but a sawtooth wave, a sine wave, a trapezoidal wave, or the like may be used instead.

[Effects of the Invention] As described above, according to the present invention, the modulation time of the recording beam is controlled according to the analog image signal obtained by converting the digital image signal into the analog image signal and the analog pattern signal. Tones can be obtained at high speed.

Further, since a pattern signal having an extreme value at substantially the center of one cycle is used as the analog pattern signal, the pulse width obtained by modulation is not at the beginning of one cycle but at `` almost the center of one cycle '' It grows from a certain "extreme" position. Thereby, the spatial frequency of the reproduced image becomes constant, and a high-gradation image can be obtained.

In addition, since the intensity of the recording beam is controlled in accordance with the digital image signal whose density has been corrected, the apparatus can be downsized and the influence of noise and the like can be reduced as compared with a device in which the recording beam intensity is controlled in accordance with the analog image signal. It is hard to receive. Furthermore, the above-described pulse width modulation and intensity control of the storage beam have an effect that density correction caused by a difference in characteristics between models and apparatuses can be easily performed.

[Brief description of the drawings]

FIG. 1 (a) is a block diagram showing a density converter of the image forming apparatus according to the embodiment, FIG. 1 (b) is a timing chart showing an example of a drive current waveform of a laser diode LD, and FIG. 2 (a). FIG. 2 is a diagram showing a relationship between a current control voltage Vc and a constant current Il in the constant current source 4, and FIGS.
FIG. 2D is a diagram showing various conversion tables between VD _{0 to} VD ₅ and output data R _{0 to} R _5. FIG. 2D is a diagram showing a relationship between a beam lighting pulse width Pwl and a potential Vd on the photosensitive drum. FIG. 3 is a diagram showing the basic configuration of a conventional laser beam printer, FIG. 4 (a) is a block diagram of a pulse width modulator usable in the laser beam printer, and FIG. 4 (b) is image density data. VD ₀ to VD ₅ to as diagram showing a relationship between the image density signals DA converted D / a, Fig. 4 (c) and (d) are timing Chiya over preparative showing the pulse width modulation by a triangular wave signal. In the figure, 1 and 6 are D / A converters, 2 is a pattern signal generator, 3 is a comparator, 4 is a constant current source, 5 is a read-only memory (ROM).

Claims (1)

(57) [Claims] An input unit for inputting a digital image signal whose image density is represented by a plurality of bits; a conversion unit for converting the digital image signal into an analog image signal; Pattern signal generation means for generating a periodic analog pattern signal, and modulation signal generation means for generating a pulse width modulation signal for pulse width modulation of a recording beam by comparing the analog image signal and the analog pattern signal, Control means for controlling the intensity of the recording beam according to a value obtained by correcting the density of the digital image signal.